A sample head of the type mentioned above is generally known, for example from U.S. publication "Review of Scientific Instruments", vol. 47, no. 12, December 1976, pages 1486 to 1488, from U.S. publication "Journal of Magnetic Resonance" 43 (1981), pages 399 to 416, and from U.S. Pat. No. 4,446,431.
The known sample heads are useful in carrying out nuclear double-resonance experiments. In the case of such experiments, one usually excites and/or observes protons (.sup.1 H) or fluorine (.sup.19 F) as a first kind of nuclei, while an isotope of nitrogen (.sup.15 N) or of phosphorus (.sup.31 P) or of carbon (.sup.13 C), for example, is excited and/or observed as a second kind of nuclei. The second kind of nuclei is generally described as "X nucleus".
The excitation frequency for protons (.sup.1 H), for example, may be 400 MHz with modern high-field nuclear resonance spectrometers. The excitation frequency (related to the same magnetic field strength) is then approx. 40.5 MHz for the before-mentioned isotopes of nitrogen (.sup.15 N), 162.0 MHz for the isotopes of phosphorus (.sup.31 P) and 100.5 MHz for the isotopes of carbon (.sup.13 C). The entire frequency range within which such double-resonance experiments are carried out, may vary between 40 and 400 MHz, but may in extreme cases even start below 40 MHz, depending on the particular kind of nuclei.
As will be explained in more detail further below, with reference to the embodiments of the invention, as compared to the prior art, the sample heads of the prior art are designed in such a way that the electric network of the sample head, comprising both the measuring coil and the r.f. line, has its equivalent circuit diagram optimized for the higher frequency, viewed from the first input terminal, and for the lower frequency, viewed from the second input terminal. With respect to the lower frequency, however, this can be achieved only for a relatively narrow range. A large frequency range, such as that resulting from the before-mentioned examples, with 40 MHz for .sup.15 N to 162 MHz for .sup.31 P, cannot be achieved for the lower frequency with the known sample heads, and this the less if the first frequency is 360 MHz or more. If, for example, the electric network of the sample head, viewed from the second input terminal, is rated for relatively low frequencies, the network would, in the case of higher frequencies, show such considerable additional inductances not employed by the sample substance that losses would be encountered, or only poor efficiency would be achieved, or it would be impossible altogether to attain the highest frequency (for example 162 MHz).
On the other hand, in the case of a configuration where the network of the sample head, viewed from the second input terminal, is rated for relatively low frequencies (for example by reducing drastically the inductance of the measuring coil), relatively poor properties would result in the lower range of the lower frequencies for the second kind of nuclei, and for the first kind of nuclei as well.
Now, it is the object of the present invention to improve a sample head and, thus, a method of the type described above in such a way as to enable nuclear resonance measurements, where signals can be irradiated and/or received via the second input terminal over a very broad frequency range, to be performed using one and the same sample head, with only certain minor commutation measures. Related to a .sup.1 H frequency of, for example, 400 MHz, the frequency range for the second signal fed in and/or received via the second input terminal is to span from a lower frequency of 40 MHz for .sup.15 N, which is lower by one order of magnitude than the .sup.1 H frequency, to an upper frequency of 162 MHz for .sup.31 P, which is somewhat lower than half the .sup.1 H frequency.